This invention relates to the chlorination of aromatic compounds. More particularly, it relates to chlorination methods and catalyst compositions capable of producing predominantly p-chloro isomers.
Chlorination of such aromatic compounds as toluene and xylenes is a known reaction affording useful compounds. The most useful of these compounds for many purposes are the p-chloro compounds. p-Chlorotoluene, for example, is an intermediate capable of conversion into many useful chemicals. The same is true of p-chloro-o-xylene; i.e., 4-chloro-1,2-dimethylbenzene. A principal utility of the latter compound is its conversion by oxidation to 4-chlorophthalic acid, which is in turn an important intermediate in the production of polyetherimides.
However, the production of these highly desirable chlorinated compounds is complicated by the simultaneous production of numerous not so desirable by-products. Thus, chlorination of toluene and xylenes produces the p-monochloro isomer in admixture with other isomers such as o-chlorotoluene and 3-chloro-1,2-dimethylbenzene. In addition, numerous polychlorinated products are ordinarily obtained.
Many of the known methods for chlorination of aromatic compounds employ elemental chlorine in combination with Lewis acids, chiefly the Lewis acids known in the art as useful in Friedel-Crafts reactions including alkylation and acylation. The use of such catalytic species as ferric chloride, antimony trichloride, antimony pentachloride, zinc chloride and aluminum chloride is conventional.
However, the use of such catalysts in no way resolves the problems of isomer distribution and polychlorination. Various publications, including many U.S. patents, go further in describing mixed catalyst systems in which another catalyst component is an organosulfur compound. The organosulfur compounds disclosed in these publications are of very diverse structures. Many of them are phenoxathiins or thianthrenes. Illustrative patents with such disclosures are U.S. Pat. Nos. 3,226,447, 4,031,142, 4,031,147, 4,190,609, 4,250,122, 4,289,916, 4,647,709 and 4,925,994. Lewis acid proportions employed according to many of these disclosures are quite low, typically on the order of 0.01% by weight based on aromatic compound being chlorinated.
U.S. Pat. No. 4,017,551 discloses the chlorination of benzene or monochlorobenzene in the presence of iron-free manganese chloride. No second catalyst component is disclosed.
In European patent application 126,669, a further class of catalyst systems is disclosed. It comprises a Lewis acid known to be useful for this purpose, preferably a chloride of iron or antimony, in combination with an N-substituted phenothiazine. Among the substituted phenothiazines disclosed is phenothiazine N-carbonyl chloride. It has been found, however, that such combinations are still somewhat deficient in that they tend to form polychloro compounds and monochloro compounds other than the p-isomer.
Soviet patent 654,602 discloses the chlorination of toluene in the presence of a chloride of copper, cobalt or nickel on a solid support. The chlorination requires vapor phase conditions and temperatures in the range of 400–450° C. Such conditions are far from ideal for commercial practice.